The present invention has particular utility and important advantages as applied to aircraft navigation, and hence much of the discussion herein relating to the background of the invention and to the objects and advantages thereof, as well as the detailed description of a particular embodiment of the invention, will be directed toward the aircraft navigation aspect of the invention. Nevertheless, as will become apparent hereinafter, the present system and/or portions thereof are equally well suited for a variety of other types of navigation, and accordingly the present system and various inventive aspects thereof are not limited to use in connection with aircraft navigation.
Early aircraft navigation systems were principally of the "point source" type, and these point source navigation systems still are by far the most widely used type systems in aircraft navigation today. Point source navigation systems include such systems as "ADF" (automatic direction finding equipment); marker beacons; "VOR" (VHF omnirange stations); and "DME" (distance measuring equipment).
The ADF spectrum is generally from about 190 KHz to about 1750 KHz, and generally includes LF (low frequency) and MF (medium frequency) non-directional beacons and AM commercial broadcast stations.
Marker beacons set at 75 MHz have been operational for many years along U.S. federal airways, but are currently being phased out as an en route aid. They are still standard along instrument landing approaches.
VOR (sometimes referred to as "Omni" stations are internationally standardized for use by aircraft, in the frequency range of 108 MHz to 118 MHz. These stations provide bearing information relative to the respective stations, and the techniques have been greatly refined in recent years in an attempt to reduce site errors and ambiquities. VOR navigation is probably the most widely used form of aircraft navigation throughout the world today.
DME is a pulse-ranging system for aircraft, in the 960-1215 MHz band, and involves pulsed transmission from the aircraft and a return pulse from a ground station after a fixed delay.
VOR and DME are now frequency colocated to form a single site area coverage system, which is a Rho-Theta area coverage grid system. For each VOR frequency there is paired DME frequency, and a common channel selector is provided in compatible airborne equipment. The U.S. VORTAC, for example, is a VOR/DME system utilizing the DME function of the military TACAN (UHF tactical air navigational aid) system for distance measurement.
These point source type navigation systems, while useful in making approaches, are generally inadequate for en route navigation, and particularly for navigation on a worldwide basis which involves flying over great expanses of water and large primitive land masses.
Similarly, the area navigation combinations of such point source navigation systems, as for example VOR/DME, are generally inadequate, and involve serious problems. Thus, these systems utilize VHF and/or UHF, both of which are very limited in range, and are generally "line of sight". Accordingly, each such station only covers a very limited surrounding area, so that literally hundreds of stations are required to cover even the limited area of the United States. Even then, for reliability, due to the likelihood of station failure, even that number must be multiplied with backup equipment and overlapping station coverage. Such point source type area navigation equipment requires manual station selection, and frequent reselection because of the short range. Also, such equipment is generally subject to station errors and obstructions.
Nevertheless, for lack of a better system prior to the present invention, this VOR/DME type area navigation equipment is currently being widely implemented by the U.S. Government on a national basis.
One approach which has recently come into some use in commercial aviation to give more flexibility in area navigation is the inertial navigation system. This is a true "area" navigation system of the dead reckoning type, wherein an airborne computer provides position, ground speed, heading, time and distance to go to destination or a selected waypoint, and the like. The computer is fed inertial information from a series of accelerometers and gyros. Such inertial navigation systems are extremely expensive and are very heavy and bulky due to the nature of the sensors, and they are therefore only suitable for very large commercial and military aircraft of the intercontinental type. Inertial navigation systems also have the disadvantage of being subject to cumulative errors from the point of departure. Accordingly, such inertial systems appear to have only very limited usefulness in aircraft navigation.
Another group of navigation systems employed for area navigation is the "hyperbolic" group of navigation systems wherein navigational position fixes are made by time referencing signals from a plurality of widely spaced ground stations in a hyperbolic coordinate system related to the ground stations. Such hyperbolic-type navigation systems have the advantage of utilizing low frequencies, generally on the order of from about 10 KHz to about 130 KHz, which greatly extends the station range from hyperbolic systems as compared to point source area navigation systems, thereby permitting navigation over large areas with the use of only a very few stations. Hyperbolic navigation systems also have the advantage that they have great inherent accuracy because of the good frequency stability that can be produced in this low frequency range, although such accuracy depends entirely upon the capability of the system to produce close synchronization between transmitted signals and receiver reference signals. Hyperbolic navigational systems also have the advantage that, assuming the transmitted signals and receiver reference signals can be accurately synchronized, errors are generally not cumulative in nature.
One of the major problems in connection with prior art hyperbolic navigation systems is that very complicated and expensive hardware has generally been required in order to provide the necessary signal synchronization. Principal prior art hyperbolic systems include "Decca", "Loran", and "Omega" systems, all of which involve such complex and expensive hardware that they are primarily useful only for ship navigation, and not practical for aircraft navigation.
The Decca system requires a "master" and three "slave" ground stations for each area of coverage, with the accompanying undesirable multiplication of hardware. Decca is in the 70 KHz to 130 KHz frequency range, which is considerably above the 3 to 30 kilohertz VLF range and therefore does not have the advantage of great undistorted range of a VLF system.
Loran requires a master and two slave ground stations, all of which are atomic clock controlled. The Loran system utilizes a 100 KHz frequency, which is undesirably high, and in order to overcome the resulting ionospheric reflection problem, Loran utilizes a special pulsed waveform and gating.
The Omega system has the advantage of being in the true VLF frequency range, between about 10 KHz and about 14 KHz, but has the disadvantage of requiring an atomic clock on board which was initially started in synchronism with the ground station clocks.
These prior art hyperbolic navigation systems are intended to provide absolute positioning, rather than dead reckoning with initial and intermittent position fixing, and such absolute positioning requirement necessitates the excessive and expensive hardware such as atomic clocks, master and slave stations, and the like, employed in the systems.
There have been recent efforts to avoid the use of atomic clocks on board and to reduce the hardware requirement in a hyperbolic navigation system by having receivers with oscillator twelve adapted to be phase-locked to respective transmitter station signals. However, such efforts have generally resulted in equipment having a navigational capability that is unduly limited and uncertain. Such prior art efforts in this direction could not accommodate random area navigation, but required pre-selection of the particular route to be flown between two points, and then prior to take-off the time signals and differences to be utilized had to be looked up in a large book of tables. Only a fraction of the existing VLF transmitting stations could be used which had frequencies capable of being matched by binary division. Lock-on of the receivers to respective station signals was very slow, and under extreme temperature conditions lock-on would not occur at all. Such prior art equipment had no capability for station averaging or diurnal shift correction, or the like. Because of the restricted number of VLF stations that could be used in such system, and the requirement of a pre-selected route, it was not generally suitable for navigation on a worldwide basis.
A problem in connection with all of these prior art hyperbolic-type navigation systems has been to convert the hyperbolic lines of time/position into a meaningful display, and particularly to provide such conversion in a sufficiently compact package for widespread availability and so as to be suitable for use in other than ships or large aircraft; i.e., suitable for use even in small aircraft, and in ground vehicles.
The VLF frequency range is generally not subject to shadowing, and will permeate areas in the middle of numerous obstructions, as for example city streets despite the presence of tall buildings and other obstructions. Accordingly, the VLF frequency range is inherently particularly suitable for use in ground vehicle location and control (which is currently handled principally by VHF equipment that is particularly subject to interference from physical obstructions). However, the prior art VLF navigation systems have generally been much too cumbersome and expensive for such purpose.